US lumber futures rose by the most allowed by exchange rules following Canfor's announcement that it would further curtail output at its mills in Canada due to declining wood demand.  Canfor, one North America's biggest lumber producers, had already cut back output at mills in British Columbia. It said late Monday that it would extend curtailments to its facilities in Alberta and into the new year because of "very weak market conditions."  Prior to Tuesday's pop, lumber futures had dropped to below $400 per thousand board feet for the first time since June ...